Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent or Auto-Lock, Retard, and Advance in the specified order. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. If a cam phaser is held at the mid-lock position with the locking pin engaged, and the engine controller commands the adjustment of the cam phaser to a new position, the locking pin may become side-loaded in the lock pin housing in the event that actuation of the cam phaser is attempted before the locking pin is fully disengaged from the phaser. This scenario may prevent or severely delay the execution of phasing requests, which may result in degraded engine performance. In addition, undue stress may be introduced to the locking machinery.
In one example, the issues described above may be addressed by a method, comprising: in response to a command for phasing a cam torque actuated variable cam timing phaser from a locked position, jumping a spool valve from a detent region to outside a null region, and ramping the spool valve through the null region while monitoring for cam timing movement away from a locked position. In this way, the locking pin may reliably disengage before a position of the cam phaser is changed, thereby reducing side-loading of the locking pin.
As an example, while a cam phaser of a VCT device is held in the mid-lock position with the locking pin engaged, an engine controller may command a phase adjustment to an advance position. In response to the phase adjustment command, a solenoid duty cycle of the spool valve may be controlled to reduce phaser actuation while the locking pin is being disengaged. In particular, because the desired phase position is an advance position, the duty cycle may first jump from the auto-lock region to a position slightly retarded from the null region. It may then be slowly ramped upward toward the advanced region, through the null region. The ramping may be continued until phasing motion is detected, the phasing motion indicative of the release of the locking pin.
In this way, the locking pin of a cam phaser may be disengaged only when the duty cycle is commanding minimal amounts of phase adjustment, if any amount at all. Additionally, the locking pin is ensured to be disengaged before the duty cycle resumes normal phasing operations. Thus reliable disengagement of the locking pin is achieved and side-loading due to phase adjustments is reduced during disengagement. Overall, valve timing control is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.